The formula of the purple colour formed in Lassaigne's test for sulphur using sodium nitroprusside is

We need to identify the formula of the purple-coloured compound formed in Lassaigne's test for sulphur using sodium nitroprusside. In this test the organic compound is fused with sodium metal to form sodium sulphide $$Na_2S$$ if sulphur is present.

$$2Na + S \rightarrow Na_2S$$

The sodium extract is then treated with sodium nitroprusside solution $$Na_2[Fe(CN)_5(NO)]$$ and the sulphide ions react to form a purple-coloured complex:

$$Na_2S + Na_2[Fe(CN)_5(NO)] \rightarrow Na_4[Fe(CN)_5(NOS)]$$

In this reaction, the $$S^{2-}$$ ion reacts with the $$NO^+$$ group in nitroprusside to form the thionitrosyl group $$NOS^-$$, resulting in the purple-coloured complex $$Na_4[Fe(CN)_5(NOS)]$$.

Option A: $$Na_4[Fe(CN)_5(NOS)]$$ — This is the correct formula of the purple complex formed in Lassaigne's test for sulphur. Option B: $$NaFe[Fe(CN)_6]$$ — This is Turnbull's blue, not related to the sulphur test. Option C: $$Na[Cr(NH_3)_2(NCS)_4]$$ — This is Reinecke's salt. Option D: $$Na_2[Fe(CN)_5(NO)]$$ — This is sodium nitroprusside itself (the reagent, not the product).

Hence, the correct answer is Option A: $$Na_4[Fe(CN)_5(NOS)]$$.